Since Adam and Eve, fruit has been universally recognized as a highly desirable food. Further, the need for consuming significant quantities of fruit for nutritional purposes is well documented. One reason the public does not consume the recommended quantities of fresh fruit is spoilage; fruit is highly perishable. Thus, the fruit is typically processed to extend its life by canning, freezing or by various evaporative approaches such as sun drying (raisins), hot air drying (dried fruit), freeze drying (blueberries for dry cereal), frying (sliced bananas), spray drying (fruit powders) and dehydration to very low moistures (for dry cereals). Each of these processes has its strengths and weaknesses:
Sun drying in the desert works well in California apricots but not in Washington apples, Michigan cherries or Florida tomatoes;
Hot-air drying makes soft fruit lacking the crispness of fresh fruit and can yield to mold and yeast spoilage over time;
Freeze drying is an elegant way to dry fruit, but it is very expensive (an order of magnitude greater than fresh fruit weight-for-weight), limiting it to high-margin products or small percentages in foods;
Frying imparts fat, offsetting the nutritional value and flavor of fruit;
Spray drying of fruit, limited mostly to pulp-free juices, requires a carrier, such as maltodextrin, that limits the fruit content.
Dehydration to moisture less than 3% depletes volatile flavors and makes a crispy but hygroscopic fruit that rapidly absorbs humidity and can become tough and hard.
The potential for providing other types of fruit products is limited by the processing difficulties associated with fruit compared to other food products. For example, extrusion technology is employed extensively throughout the grain processing industry (but not in fruit processing) to cook grain-based and soy foods because the process is energy efficient, reliable, and sanitary. Major industry segments utilizing extrusion cooking include ready-to-eat cereals, snacks, pet foods, industrial pre-gelled flours, and many others.
A cooking extruder is typically a screw machine that accepts free-flowing grain meal or flour as in-feed material into a progressively reducing, spiral-screw cavity. As the material progresses along the screw or multiple screws of the extruder, the in-feed material is hydrated by water injection (for example, from a 10-12% in-feed moisture to a 15-30% dough moisture), and the moistened material is compressed and heated by friction to “pressure cook” the extrusion dough with the moisture encapsulated as steam. Typically, extrusion in-feed materials must be uniformly free flowing and finely granular, both hallmarks of milled grains such as corn meal, wheat and rice flours, etc. By contrast, fruit products (i) are not as free flowing, causing stoppage of the in-feed material (except in forms too liquid for extrusion cooking), (ii) are often heterogeneous in particle size or granulation, and (iii) are hygroscopic when dried. These characteristics during the 55-year history of food extrusion processing have virtually eliminated extrusion cooking from consideration when processing fruit.
Starches, flours and meals milled from grain have traditionally been used by those skilled in the art of extrusion to manipulate texture and density of cereal foods, such as ready-to-eat breakfast cereals; snacks, such as corn puffs and onion rings; pet foods, such as kibbled dog foods; and many other foods. Starches are long-chain carbohydrates that, when gelatinized by extrusion cooking, form films capable of trapping gas (air and steam) in thin-walled, honey-comb-like structures, aerating the product (“puffing”) and reducing the density. In traditional practice, fruit powders have been added in low percentages (for example, Kellogg'® Apple Jacks®) to impart fruit flavor, color or marketing sizzle to starch-based puffed foods.
Some limited attempts to use extrusion technology for fruit have met with mixed results. Typically, the fruit content is severely limited, and in some cases eliminated, so that the technology is virtually identical to the extrusion cooking of grain without fruit. In such cases, the principal ingredients are starches, sugars, gels, gums, flavors and colors, with a small percent of dried or powdered fruit.
The technology for producing high fruit content food products by extrusion is limited. For example, a recent application of extrusion technology to fruit processing has copied starch and grain-based extrusion practices associated with the in-feed material metered into the extruder. U.S. Pat. No. 6,027,758 describes a traditional use of starches that are added as gelling agents to control density and texture of extruded foods, including those composed largely of fruit, the apparent focus of this reference. In a number of pre-extrusion steps, this reference first drum dries the starting material, fruit puree, down to 6% moisture, creating fruit flakes or coarse granules; then grinds the dried fruit solids to a powder form intended for homogeneous, steady metering into the feed section of an extruder; and finally adds water or other liquids back into the barrel of the extruder to facilitate cooking and to prevent mechanical seizure of the extrusion screw. The approach applies traditional grain extrusion practices (drying the in-feed to low moisture, grinding to uniform granulation, and then rehydrating in the extrusion barrel) to a fruit starting material, fruit puree.
In one portion, the reference describes an effect of heat on gelatinization that is well known in extrusion technology and in most other food processing applications in which carbohydrates are cooked. Starch gelatinization occurs under conditions combining water with a temperature of at least 160° F., usually with moderate shear. When processing below the gelatinization temperature, the starch in the reference's fruit product is predictably ungelatinized and the product is dense. As the fruit product exits the extruder, the texture is soft and chewy, like the gels of the reference having a water activity level of at least 0.58. At higher temperatures, gelatinization of the starch occurs by the well-known mechanism, forming a film capable of trapping air and steam. Further, the reference makes no mention of drying technology or equipment to achieve low moisture levels after extrusion, and the reference presents data describing finished products in the moisture range of 19-25%, over three times the upper range for crisp starch products. The products of the reference are high or intermediate moisture gels similar to those used in popular breakfast bars such as Pop Tarts and the like or those with starch content to produce different textures.
A problem with this reference that may preclude it offering a commercially viable approach is that, unlike grain flours or meals, the dried-and-ground fruit powder is hygroscopic, such that it remains free flowing and non-tacky only for a short while, and tends over time to build up on the sidewalls of the handling equipment, most notably in the moist feed section of the extruder, inhibiting sanitary and efficient processing. Such hygroscopic in-feed materials require specialized handling in a commercial production setting and are avoided when possible. To illustrate how industry deals with the hygroscopic nature of fruit powders, they are typically packed only in small quantities in multilayer laminate film pouches that include aluminum foil or Mylar or some other absolute vapor barrier that prevents atmospheric humidity from creating hygroscopic caking of the fruit powder prior to use. Thus, the simple application of a starch and grain-based extrusion technology to the manufacture of products with a high fruit percentage is not readily suitable for commercial production.
In another example of extrusion technology, U.S. Pat. Application No. 20040022901 admixes specially processed crisped rice that has been manufactured in a traditional application of extrusion technology for the cooking of rice flour with a non-extruded fruit product. In this case, only the grain fraction, i.e., the rice flour, is extrusion processed, while the fruit is simply admixed with the extruded crisped rice.
These problems and the attempted solutions using starch and grain-based extrusion principles illustrate the need for a new product, system, and process for high percentage fruit processing with extrusion technology that significantly departs from prior teachings. Thus, there remains a need for high-fruit-content foods produced by extrusion technology from commercially available fruit ingredients that can be handled practically in typical food plant equipment.